High molecular weight polyesters and particularly polyesters and copolyesters of glycols and terephthalic or isophthalic acid have been available for a number of years. These are described inter alia in Whinfield et al, U.S. Pat. No. 2,465,319 and in Pengilly, U.S. Pat. No. 3,047,539. These patents disclose that the polyesters are particularly advantageous as film and fiber-formers.
With the development of molecular weight control, the use of nucleating agents and two-step molding cycles, poly(ethylene terephthalate) has become an important constituent of injection moldable compositions. Further, poly(1,4-butylene terephthalate), because of its very rapid crystallization from the melt, is uniquely useful as a component in such compositions. Work pieces molded from such polyester resins, in comparison with other thermoplastics, offer a high degree of surface hardness and abrasion resistance, high gloss, and lower surface friction.
Furthermore, in particular, poly(1,4-butylene terephthalate) is much simpler to use in injection molding techniques than poly(ethylene terephthalate). For example, it is possible to injection mold poly (1,4-butylene terephthalate) at lower mold temperatures of from about 230.degree. C. to 260.degree. C. to produce highly crystalline, dimensionally stable moldings in short cycle times. On account of the high rate of crystallization, even at low mold temperatures, no difficulty is encountered in removing the moldings from the molds.
It is also known to add certain additives to thermoplastic polyesters to enhance or provide certain properties. For example, Rein et al, U.S. Pat. No. 3,405,198, discloses the use of polyethylene in poly(ethylene terephthalate) as an impact modifier. Holub et al, U.S. Pat No. 4,122,061, discloses polyester compositions which comprise a poly(1,4-butylene terephthalate) resin, a poly(ethylene terephthalate) resin, a fibrous glass reinforcement, alone or in combination with a mineral filler and, as an impact modifier therefor, a polyolefin or olefin based copolymer resin including polyethylene and propyleneethylene copolymer. Cohen et al, U.S. Pat. No. 4,185,047, disclose the use of high pressure low density polyethylene in thermoplastic polyester compositions, particularly poly(ethylene terephthalate) and poly (1,4-butylene terephthalate) for improved mold releasability. Kohler, et al, U.S. Pat. No. 4,666,972, teaches a thermoplastic molding composition comprising a polyalkylene terephthalate, a graft polymer and a fluorinated polyolefin. This reference indicates that molding compositions are considerably tougher, and have much higher moduli of elasticity than molding compositions which are free from fluorinated polyolefins.
It is further known that dispersing poly(tetrafluoroethylene) in poly(butylene terephthalate) produces a molded compound with increased resistance to wear. For example Hilt, et al, German Offen. 2,452,329, teaches a composition containing 75-99 parts PBT and 1-25 parts poly(tetrafluoroethylene), which is dispersed in the polyester in the form of the discreet particles of average size 0.15-0.80 microns.
It has been discovered that the wear resistance properties of polyesters that are compounded with fluorinated polyolefins are improved if there is not a homogeneous dispersion of fluorinated polyolefin throughout the composition but rather if there is a disproportionate concentration of such polyolefins located within the subsurface of the polyester/polyolefin composition. Normally, the fluorinated polyolefin will be homogeneously dispersed throughout the blended composition. In this regard, it has been further discovered that a combination of a hindered phenolic compound and a specified ester compound, when used in conjunction with a polyester and fluorinated polyolefin, functions as a migrator in the blended composition to thereby facilitate the location of a disproportionate amount of fluorinated polyolefin within the subsurface of the blended compound. This tends to augment the wear resistance properties of the fluorinated polyolefin so that less of this compound is needed in order to achieve the desired wear resistance properties of the resulting polyolefin composition. The term "migrator" or "migrator package" is used herein to indicate a component package that, when utilized in conjunction with a polyester and fluorinated polyolefin blend, produces a resulting composition that has a greater concentration of fluorinated polyolefin component located within the subsurface (that is, defined herein as being within about three microns, and preferably within one micron of the surface) of the composition then similar compositions compounded without the migrator package materials. It should be noted that the exact mechanism whereby the fluorinated polyolefin material tends to be located within the subsurface of the composition in a higher concentration than would be expected is not considered to be part of the present invention and no claim is made as to any particular theory of exactly how this may be accomplished on a molecular level, other then that the location of the fluorinated polyofefin is clearly causally related to the presence of the migrator package in the blend.